If someone just downloaded old music, say from 20 or more years ago,but bought all current music, is this still considered illegal even though the artists aren't selling any more records anyway?
Yes it is still illegal because someone still owns the copyright and someone still gets a percentage of the profits from each sale. I believe that copyright lapses after 60 years and the work falls into the public domain, but I also understand that the copyright can be renewed. So early works by someone like Frank Sinatra have no doubt been re-copyrighted.
Indeed, there is some discussion about the 60 year copyright rule as were getting into an era where the Beatles, Elvis catalogues will ineffect become free to all. Very greedy if you ask me considering all the money they have generated.
50 years, not 60. Anything released before 1 January 1954 is now in the public domain in the UK, though many of the songs themselves will still be under copyright. For instance, Bing Crosby's recording of "White Christmas" has been in the public domain for several years now - Bing's estate won't get anything from it, but Irving Berlin's estate will still get royalties on the song, because intellectual copyright lasts longer than mechanical copyright.
I understand this is a bit of a legal hot potato because of what jenstar mentions. Apparently the copyright on the song (publication) is much longer (100 years?) than the performance and this is a source of some controversy.